Discharge device and electrode



Oct. 19, 1954 H. L. STEELE, JR I 2,692,348

DISCHARGE DEVIC;=E AND ELECTRODE Filed Jun 19, 1951 INVENTOR BY v X 9.

' ATTORNEY v I Patented Oct. 19, 1954 DISCHARGE DEVICE AND ELECTRODEHoward L. Steele, Jr., North Caldwell, N. J., as-

signor to Westinghouse Electric Corporation, East Pittsburgh, Pa., acorporation of Pennsylvania Application June 19, 1951, Serial No.232,361

2 Claims.

This invention relates to lamps and, more particularly, to electrodesfor low pressure discharge devices of the positive column type, such asfluorescent lamps.

Manufacturers of low pressure discharge de- 1 vices of the positivecolumn type, such as fluorescent lamps, are continually trying toincrease the luminous efiiciency, namely, lumens or light output perWatt of power input, of the product. In this attempt, it has been foundthat the gas atoms surrounding the electrodes of a fluorescent lamp donot act as efiiciently in generating light as do the atoms in the centerof the lamp.

During the half cycle of operation on alternating current in which oneelectrode of a fluorescent lamp is functioning as an anode, a deficiency of positive ions occurs near this electrode or anode, and anegative space charge builds up there about. An anode voltage dropresults from this space charge and is a measure of the energy which mustbe imparted to the random electrons, in order to overcome the repellingforce of the space charge and to accelerate enough of these randomelectrons to the anode to supply the lamp current. It has been foundthat to efliciently draw the required lamp current from the randomelectrons in the positive column, and thus substantially minimizeaccelerating anode voltage drop, an anode of proper size and shape isrequired.

The size of the anode can be determined from the values of the randomelectron current as measured by Langmuir probe methods. If the anodearea is large enough, the accelerating anode voltage is theoreticallynot necessary. Zero anode drop should be obtained when the product ofthe anode area and the random electron current equals the lamp current.If the anode area is above this critical value, a negative or retardinganode drop should result.

Further, the larger the anode area, the better is the luminousefiiciency near the anode. However, any large anode located in front ofan emitting electrode casts its shadow on the phosphor coating of thelamp envelope wall. This invention involves a compromise between ananode of large area and objectionable shadow caused by such an anode. Byplacing an anode having an area of the above-mentioned critical value onthe axis of the envelope, the phosphor dark area produced by the anodeshadow is not objectionable. The anode should be shaped to fit theequi-potential lines in the discharge which are concave toward theanode.

Hence, it has been found advantageous, according to my invention, toprovide in a discharge device a bullet-shaped anode having a surfacearea not smaller than the operating current of the device divided by therandom current density in the device at the anode on the axis of thedischarge where random electron current dens'ity is greatest, and infront of an electron-emissive electrode adjacent said axis, toeifectively reduce anode drop and increase luminous efficiency. Becauseof the anodes position in "the discharge, any 253? Angstrom unitsradiation which is produced near the anode has a straight line path tothe phosphor on the lamp wall, and is not lost within the anode.Further, the bullet-shape of the anode fits the equi-potentia1lines inthe discharge which are concave toward the anode.

In its general aspect, the present invention .has the object ofovercoming the aforementioned disadvantages of the prior art lowpressure discharge devices of the positive column type,

The principal object of the invention is to provide a bullet-shapedanode on the axis of the discharge and in front of a conventionalelectron-emissive electrode of a low pressure discharge device of thepositive column type, to effectively reduce anode voltage drop and toincrease luminous efiiciency.

Another object is to provide a bullet-shaped anode for a low pressuredischarge device of the positive column type which as much as possibleavoids augmenting the dark area of the phosphor caused by the anodeshadow.

An additional object is to provide a bulletshaped anode for a lowpressure discharge device of the positive column type which fits theequi-potential lines in the discharge of the device.

A further object is to provide a bullet-shaped anode for a low pressuredischarge device of the positive column type which permits 2537 A. U.radiations produced near the anode to travel in straight lines to thephosphor coating on the envelope of the device.

Other objects and advantages of the invention will appear to thoseskilled in the art to which it appertains as the description proceeds,both by direct recitation and by implication from the context.

Referring to the accompanying drawing, in which like numerals ofreference indicate similar parts throughout the several views:

Fig. 1 is a fragmentary elevational View of -a discharge lamp with partsin axial section, and showing the bullet-shaped anode of my invention;

Fig. 2 is a. transverse sectional view on line IIII of Fig. 1, in thedirection of the arrows;

Fig. 3 is a perspective view of one of the mounts of the lamp of Fig. 1;

Fig. 4 is an axial sectional view of the bulletshaped anode of myinvention;

Fig. 5 is a perspective view similar to Fig. 3, but showing analternative embodiment of the bullet-shaped anode of my invention;

Fig. 6 is a sectional view, similar to Fig. 4, but showing thealternative anode embodiment of Fig. 5;

Fig. 7 is a perspective view similar to Figs. 3 and 5, but showing afurther alternative embodiment of the bullet-shaped anode of myinvention.

Referring now to the drawing in detail, and particularly to Fig. l, thereference numeral I designates a low pressure discharge device of thepositive column type. While a fluorescent lamp has been shown as anembodiment of such a device, it will be understood that use of thisinvention is not restricted to fluorescent lamps. This device or lampIll comprises a vitreous envelope II suitably coated with a phosphor andcontaining an ionizable medium, for initiating and sustaining adischarge, such as an inert gaseous fill at low pressure admixed withmercury vapor, and oppositely positioned electrode 'mounts I2hermetically sealed in each end thereof. A suitable base I3 is affixedto the envelope at each end.

The mount I2 shown in Figs. 2 and 3 consists of a filamentary cathodeI4, supported adjacent the axis of the lamp discharge by leading-inconductors I and I6 sealed through the press I! of a vitreous stem I8,and a bullet-shaped anode I9.

Each bullet-shaped anode I9, the cylindrical portion of which is aboutin diameter, with a semi-spherical end portion or dome T36" in radius ateach end, and approximately /4" long, is made of a suitablereasonably-low work function material, such as nickel or misch metal. Itis hollowed out to reduce its mass and has its outer portion formed as ahollow neck-like projection 20 (Fig. 4) for joining to an L-shapedmetallic conductor 2 I.

Each anode is mounted on the axis of discharge where the electronconcentration or random electron current is a maximum, by means of saidconnector 2|, and with the dome tip for example, about in front of itscathode I4. The connector 2I is secured, as by welding, to one of theleading-in conductors I5 and IS. The domelike forward or inner portionof each anode fits the equi-potential lines 22, shown in Fig. 1, in thedischarge of lamp ID. This mounting arrangement of the axial anodes I9,and adjacent oathodes I4 located outwardly from their anodes, minimizesthe dark area of the phosphor caused by the anode shadow, and permits2537 Angstrom units radiations produced near the anode to travel instraight line paths to the phosphor.

A suitable exhaust hole 23 (Fig. 4) in the wall of the anode permitsevacuation of the hollow interior of the anode during exhaust of saidlamp I0.

If the product of the surface area of anode I9 and the random currentdensity equals the current of the lamp ID, the anode voltage drop shouldbe zero. Examples of this critical value of anode surface area, which isnot smaller than the operating current of lamp divided by the randomcurrent density of the lamp at the anode, for a given lamp current,given gas fill, and given 4 gas fill pressure in a fluorescent lamp, aresummarized in the following table:

Random Area in Gas Fill Lamp Cur- E2533? cm. Which Gas Fill Pressurerent D ensity S hould (mm.) (ma) on the Axis Gige Zero in maJcm. mp

As an alternative embodiment, the anodes of my invention may be solidand bullet-shaped as shown in Fig. 6 and designated by the referencenumeral I9. A suitable axial hole is provided at one end of anode I9 foraflixing, as by welding or by a threaded connection, a metallic dummylead 24; which is suitably of larger diameter than the connector 25 ofFig. 3 and Fig. 4, to better support the increased mass of anode I9.Except for the use of a suitable intermediate connector 25, between theconnector 24 and one of the conductors I5 and I6, the surface area ofanode I9 is dimensionally the same, and it is mounted similarly in stemI8, as shown in Fig. 5, and functions in lamp II) in the same manner asanode I9.

In Fig. '7, there is shown a further embodiment of my invention, namelya hollow cup-like bullet-shaped anode [9 which is mounted axially onstem I8 by means of the leading-in conductor I6 with the dome tip, forexample, about 1 from the also axially mounted cathode I4. As in thecase of the other above-mentioned embodiments, the outer surface area ofanode I9 is dimensionally the same as anodes IQ of Fig. 3 and I9 of Fig.5.

Thus it will be seen from the foregoing description that I have providedbullet-shaped anodes i9, I9 or I9 on the axis of the discharge and infront of filamentary cathodes I4 of a low pressure discharge deviceIilof the positive column type to effectively reduce anode voltage dropand to increase luminous efiiciency. This arrangement of thebullet-shaped anodes in front of the adjacent conventional cathodesI4'minimizes the dark area of the phosphor causedby the anode shadow,and permits mercury resonance radiations produced near the anode totravel in a straight line path to the phosphor. The dome-like endsection of each bullet-shaped anode I9 is designed to fit theequi-potential lines 22 of the pressure discharge device IUJ Whilepreferred embodiments of my invention have been disclosed, it will beunderstood that modifications may be within the spirit and scope of theappended claims. I

I claim: 1

l. A low pressure discharge device of the positive column typecomprising a vitreous envelope having a phosphor coating thereon,oppositely disposed electrode mounts hermetically sealed to saidenvelope, and an ionizable niedium in said envelope for initiating andsustaining a discharge; at least one of said electrode mounts comprisinga stem having at least two leading-in conductors, a filamentary cathodesupported on said leading-in conductors adjacent the axis of thedischarge of said device, and a bullet-shaped anode on the axis of thedischarge on one of said leading-in conductors of said device inwardlyof said cathode and having sufficient surface area to minimize anodevoltage drop in said device, said area being larger than the operatingcurrent of the device divided by the random current density of thedevice at the anode.

2. In a low pressure discharge device of the positive column type, anelectrode mount comprising a stem having at least two leading-inconductors, a filamentary cathode supported on said leading-inconductors adjacent the axis of said device and a bullet-shaped anode onthe axis of said device on one of said leading-in conductors inwardly ofsaid cathode, and having sufflcient surface area to minimize anodevoltage drop in said device, said area being larger than the operatingcurrent of the device divided by the random current density of thedevice at the anode.

References Cited in the file of this patent UNITED STATES PATENTS NumberNumber Name Date Phillips Jan. 15, 1924 Zworykin Dec. 22, 1931 Spanneret a1 Mar. 28, 1933 Ewest June 5, 1934 Cox Nov. 10, 1936 Mayer Dec. 5,1939 FOREIGN PATENTS Country Date Great Britain printed in 1933

